1. Field of the Invention
The invention relates to motion control systems for controlling relative motion between two objects along a predetermined path of travel and, in particular, to a disk file head positioning system.
2. Description of the Prior Art
Relative motion of two objects along a predetermined path of travel according to a predetermined velocity profile and their positioning at a predetermined relative position has conventionally been controlled by circuitry employing a position transducer for indicating the position and a tachometer for measuring the velocity of said objects. The tachometer velocity is compared with a desired velocity to control the motion in a closed loop servo system and the final position is maintained at the end of said motion also by closed loop servo system in response to the position transducer output. Track crossing signals derived from the position transducer output are used in conjunction with a difference counter and non-linear digital to analog converter are used to generate the desired velocity signal.
A representative description of such a system for controlling the motion and positioning of heads relative to concentric tracks on a disk record medium may be found in an article "Design of a Disk File Head-Positioning Servo" by R. K. Oswald (IBM Journal of Research and Development, November 1974, pp. 506-512). In early disk files external position transducers, such as an optical grating and photocell arrangement or an electromagnetic resolver, were used to define the data head position over a data track. In the Oswald article, the position transducer is effectively formed by a servo head and servo position reference information on a servo disk to permit a data head, movable with the servo head, actually to follow a data track of interest.
In such prior systems, the position transducer signals have also been employed in tachometer circuitry for deriving the head velocity from a combination of integrated motor current and the differentiated position transducer signals. Such a tachometer circuit is shown in U.S. Pat. No. 3568059 (Sordello). In that patent, a differentiated discontinuous position signal is sampled in its accurate linear regions and is used to reset periodically a velocity signal output generated primarily by the integrated motor current. The sampling times are determined from the magnitude of the position signal itself and occur when the position signal is linear. The Sordello patent indicates that the derived velocity signal may be employed in a "bang-bang servo" for moving a load to a desired location.
In addition to the dedicated servo disk track following systems described in the Oswald article, there is known, in the prior art, a type of disk file in which servo information is recorded in a plurality of spaced sectors between larger data sectors of concentric information storage tracks on a disk record medium. Such a system is described in U.S. Pat. No. 3185972 (Sippel). An advantage of such a system is that the servo information is physically located in the data tracks of interest which can, therefore, be followed with greater accuracy. The servo sectors must be sampled at times defined by clock signals to enable a position error signal to be generated for track following purposes. The Sippel patent refers to but does not describe a coarse positioning system for moving the head between tracks.
Thus in the prior art, only indirect use has been made of position error signals to control motion of objects between reference positions spaced in the direction of travel. Such signals have been employed directly as control signals only to maintain the objects in a predetermined relative position.